For golf clubs, there are iron type clubs that are used mainly for a shot on a fairway, rough, bunker etc., or a tie shot from a short hole (or par 3 hole), also there are utility clubs for play, the heads of which being similar to iron clubs in the shape.
For the iron type golf club heads, stainless steal, carbon steel, titanium, or several types of alloys are widely used to form them that are from their face part to hosel part.
The iron type golf club head has a face surface for hitting a ball and a sole surface facing the ground. A hosel part is provided at a heel side of the head. A shaft is inserted into the hosel part, being firmly bonded by a bonding means such as an adhesive agent or the like.
Regarding an iron type golf club head that has an expansion part, its gravity center is low, and a distance measured from its gravity center to a face surface (or gravity depth degree) is large. The expansion part has a shape that is formed by its sole side of the head projecting rearward.
In Patent Doc. 1, it is disclosed that a balance adjustment and weight adjustment of an iron type golf club head are achieved by providing a slit that penetrates such an expansion part from the upper surface to the lower surface. Also, at paragraph [0014] of Patent Doc. 1, it is disclosed that the slit may be entirely filled with ceramic or fiber reinforced synthetic resin.
In Patent Doc. 2, an iron head is disclosed, in which a slit (or omission part) is provided at the expansion part, the slit penetrating the expansion part in the up-down direction, and the entire slit is filled with an elastic body. By providing the omission part, even if the head hits a ball with a lower part of its face, the resilience ratio of the face becomes high and the ball hitting feeling becomes excellent.
In Patent Doc. 3, an iron head is disclosed in which a recess part is provided, the recess part subsiding from the upper surface of the expansion part, and an elastic part is filled in an inlet part (or upper part) of the recess part.